The timing of female fertility can be altered by social signals from females in a social group. In rats, ovulation can be accelerated by odors from pregnant or preovulatory females and delayed by odors from lactating or ovulatory females. These odors are pheromones and may result in the synchronization or suppression of estrus within the social group, two different strategies for achieving successful reproduction under different conditions and examples of the behavioral regulation of fertility. We propose to: (1) Identify the hormonal response to odors produced during the birth cycle (pregnancy, birth, and lactation) and the estrous cycle, using a modified jugular cannulation technique. Our technique is novel, because it permits repeated sampling of several steroids and gonadotropins from intact female rats. Because this type of data has not been available previously, even our baseline data will be unique and useful. Once we have established the normal relationship of these hormones, we will be able to assess the effect of birth and estrous cycle pheromones on that relationship and distinguish among several hypothesized mechanisms for controlling the time of ovulation. (2) Determine the endocrine basis for individual differences in fertility that may affect differential reproductive success, including individual differences in vulnerability to birth cycle pheromones, reproductive behavior, and length of the reproductive lifespan. (3) Finally, we will determine whether birth cycle pheromones mediate the social suppression of postpartum fertility, a phenomenon that may have particular significance for the evolution of pheromonal regulation of female fertility.